Journal of Pre-Clinical and Clinical Research, 2017, Vol 11, No 1, 61-65 ORIGINAL ARTICLE www.jpccr.eu Interactions of retigabine with topiramate in the mouse tonic-clonic seizure model and chimney test – an isobolographic analysis Mirosław Zagaja1, Barbara Miziak2, Maria W. Kondrat-Wróbel2, Marta Andres-Mach1, Paula Wróblewska-Łuczka1, Piotr Adamczuk1, Robert Chmura3, Stanisław Jerzy Czuczwar2, Jarogniew J. Łuszczki4 1 Isobolographic Analysis Laboratory, Institute of Rural Health, Lublin, Poland 2 Department of Pathophysiology, Medical University, Lublin, Poland 3 Department of Biostatistics, Demography and Epidemiology, Institute of Rural Health, Lublin, Poland 4 Department of Pathophysiology, Medical University, Lublin, Poland Isobolographic Analysis Laboratory, Institute of Rural Health, Lublin, Poland Zagaja M, Miziak B, Kondrat-Wróbel MW, Andres-Mach M, Wróblewska-Łuczka P, Adamczuk P, Chmura R, Czuczwar SJ, Łuszczki JJ. Interactions of retigabine with topiramate in the mouse tonic-clonic seizure model and chimney test – an isobolographic analysis. J Pre-Clin Clin Res. 2017; 11(1): 61–65. doi: 10.26444/jpccr/74557 Abstract Introduction and objectives. Nowadays, one of the treatment options for patients with refractory epilepsy is polytherapy with two or more antiepileptic drugs (AEDs). Retigabine (RTG) is a novel third-generation AED with unique molecular mechanisms of action that has recently been approved as an add-on drug for the treatment of tonic-clonic seizures. To characterize types of interactions between RTG and topiramate (TPM – a second-generation AED), the maximal electro- shock-induced seizure model (MES) and chimney test in mice were used. Materials and method. In the MES model, the anticonvulsant effects of the drugs in terms of suppression of tonic-clonic seizures in male albino Swiss mice were assessed. In the chimney test, the acute neurotoxic effects of the drugs with respect to impairment of motor coordination were determined. Type I isobolographic analysis for the combination of RTG and TPM was applied to assess the anticonvulsant and neurotoxic effects in both the MES and chimney tests. Total brain concentrations of RTG and TPM were measured to exclude any pharmacokinetic interaction between drugs. Results. The type I isobolographic analysis of interaction revealed that the combination of RTG with TPM produced additive interaction in the MES test and additivity, with a slight tendency towards antagonism in terms of acute neurotoxic effects in the chimney test. Neither RTG nor TPM mutually affected total brain concentrations in the experimental animals. Conclusions. The isobolographically analyzed combination of RTG with TPM is favourable and may be recommended to some patients with refractory epilepsy. Key words maximal electro-shock, drug interaction, isobolographic analysis, Retigabine, Topiramate INTRODUCTION profiles in both preclinical studies on animals and epileptic patients [2, 4]. The most favourable combinations of AEDs are Nowadays, the efficacious treatment of patients with epilepsy those reported in preclinical studies of synergistic interaction is a challenging issue for clinicians because of the various with respect to seizure suppression and/or antagonistic types of seizures and availability of dozens of antiepileptic interaction with respect to side (neurotoxic) effects 5,[ 6]. drugs (AEDs). 30% of epilepsy patients are still inadequately Retigabine (RTG – a third-generation AED) has been treated with the currently available AEDs [1], and therefore, approved for clinical use as an add-on drug in partial these patients need the therapeutic application of two or onset seizures in adult patients with epilepsy, where other more AEDs [2]. It is obvious that every combination of AED combinations proved to be insufficient to terminate AEDs produces pharmacodynamic, pharmacokinetic or seizures [7]. RTG has a unique molecular mechanisms of both interactions. To terminate seizures in epileptic patients, action owing to properties of a selective M-current potassium clinicians are sometimes obliged to combine various channel opener and a subtype selective modulator of GABAA AEDs without any previous information about the type receptors [8, 9]. of interactions between the combined drugs. To provide In previous preclinical studies, the combination of RTG clinicians with necessary information about the perfect with valproate (VPA) exerted supra-additive (synergistic) selection of AEDs in combination, experiments carried out interaction in the mouse maximal electro-shock-induced on animals can properly assess the types of interactions seizure (MES) model [10]. The combinations of RTG with between the AEDs [3]. Overwhelming evidence indicates that carbamazepine (CBZ), lamotrigine (LTG), oxcarbazepine several two-drug combinations of AEDs displayed favourable (OXC) and phenytoin (PHT) exerted additive interaction in the mouse MES model [10–12]. The combination of RTG with Address for correspondence: Jarogniew J. Łuszczki, Isobolographic Analysis Laboratory, Institute of Rural Health, Jaczewskiego 2, PL 20-950 Lublin, Poland levetiracetam (LEV) exerted both additive and supra-additive E-mail: [email protected] interactions in the mouse MES model [13]. Additionally, Received: 30.05.2017; accepted: 08.06.2017 the combinations of RTG with CBZ, LEV, LTG and VPA 62 Journal of Pre-Clinical and Clinical Research, 2017, Vol 11, No 1 Mirosław Zagaja, Barbara Miziak, Maria W. Kondrat-Wróbel, Marta Andres-Mach, Paula Wróblewska-Łuczka, Piotr Adamczuk et al. Interactions of retigabine… have recently proved to be clinically effective as the add-on The mice receiving RTG and TPM in high doses displayed therapies in patients with partial-onset seizures [14]. ataxia and impairment of motor performance, which were The aim of this study was to perform experiments on quantified with the chimney test 17].[ In this test, the mice animals to characterize the preclinical profile of the had to climb backwards up a transparent plastic tube (3 cm combination of RTG and topiramate (TPM – a second- inner diameter, 30 cm length) within 1 min., as described in generation AED with multiple molecular mechanisms of more detail elsewhere [11, 12]. The acute neurotoxic effects action) in the MES-induced seizure model and chimney of RTG and TPM observed in the animals in the chimney test in mice by the use of type I isobolographic analysis. test were expressed as their median toxic doses (TD50 values), The choice of TPM and RTG for the combination study i.e., doses of AEDs that disturbed motor coordination in was based on the theoretical supposition that their various 50% of the tested animals. The animals received increasing molecular mechanisms of action can be mutually effective. doses of RTG and TPM to obtain an increasing percentage Furthermore, TPM is clinically used in patients with tonic- of impairment of motor coordination that allowed the clonic seizures and partial onset seizures [15]. construction of dose-response effect lines for RTG and TPM. It is widely accepted that the MES model is considered Similarly, the neurotoxic (adverse) effects of the mixture of to be an experimental model of tonic-clonic seizures and RTG with TPM were determined and expressed as the TD50 exp partial onset of convulsions in humans [16]. The chimney value, i.e., the dose of the mixture required to impair motor test is considered to be a model assessing acute adverse coordination in 50% of mice subjected to the chimney test. (neurotoxic) effects in experimental animals with respect to This experimental procedure has been described in more the impairment of motor coordination [17]. To exclude any detail elsewhere [11, 12, 19]. pharmacokinetic interactions, total brain concentrations of both, RTG and TPM were determined. Type I isobolographic analysis. Interactions between RTG and TPM in the MES-induced seizure and chimney test were isobolographically analyzed according to the methodology MATERIALS AND METHODS as described elsewhere [10–12, 20]. After determining the ED50 and TD50 values for RTG and TPM administered alone, Animals. Adult male albino Swiss mice (weighing 22 – median additive doses for the mixture of RTG with TPM (i.e., 26 g) were used in this study. All experimental procedures ED50 add and TD50 add) were calculated. Subsequently, the test described complied with the ARRIVE guidelines, approved for parallelism of dose-response effects for RTG and TPM was by the local Ethics Committee at the Medical University used independently for both the MES and chimney tests [6, in Lublin, and conformed to the Guide for the Care and 20]. In this study, two parameters were calculated: protective Use of Laboratory Animals (License No.: 28/2007). Each indices (PI) and benefit indices (BI). The PI were ratios of experimental group consisted of 8 mice. TD50 values from the chimney test and the respective ED50 values from the MES model. The PI reflected a satisfactory Drug administration. RTG (GlaxoSmithKline, Brentford, margin of safety between the anticonvulsant and neurotoxic UK) and TPM (Cilag AG, Schaffhausen, Switzerland) were doses of RTG and TPM [21]. The BI were ratios of PI exp values suspended in a 1% solution of Tween 80 (Sigma-Aldrich, (determined experimentally) and PI add values (theoretically Poznań, Poland) in distilled water and administered predicted to be additive). The BI reflected advantages for intraperitoneally (i.p.) in a volume of 5 ml/kg body weight. the combination of RTG with TPM, considering both RTG was